Merge pull request #4629 from shuujii/mrb_str_pool-can-embed-one-more-byte

`mrb_str_pool` can embed one more byte; ref #4626

src/state.c

@@ -170,7 +170,7 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
       len = s->as.heap.len;
     }
 
-    if (len < RSTRING_EMBED_LEN_MAX) {
+    if (RSTR_EMBEDDABLE_P(len)) {
       RSTR_SET_EMBED_FLAG(ns);
       RSTR_SET_EMBED_LEN(ns, len);
       if (ptr) {